I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for synchronizing base stations in a wireless communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both of which are assigned to the assignee of the present invention and are incorporated by reference. The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association in TIA/EIA/IS95-A entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, referred to herein as IS-95.
In the just mentioned patents, a multiple access technique is disclosed in which a large number of mobile station users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell base stations or cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. By using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA techniques result in much higher spectral efficiency than can be achieved using other multiple access techniques.
A method for simultaneously demodulating data that has traveled along different propagation paths from one base station and for simultaneously demodulating data redundantly provided from more than one base station is disclosed in U.S. Pat. No. 5,109,390 (the ""390 patent), entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In the ""390 patent, the separately demodulated signals are combined to provide an estimate of the transmitted data which has higher reliability than the data demodulated by any one path or from any one base station.
Handoffs can generally be divided into two categoriesxe2x80x94hard handoffs and soft handoffs. In a hard handoff, when a mobile station leaves an origination base station and enters a destination base station, the mobile station breaks its communication link with the origination base station and thereafter establishes a new communication link with the destination base station. In soft handoff, the mobile station completes a communication link with the destination base station prior to breaking its communication link with the origination base station. Thus, in soft handoff, the mobile station is redundantly in communication with both the origination base station and the destination base station for some period of time.
Soft handoffs are far less likely to drop calls than hard handoffs. In addition, when a mobile station travels near the coverage boundary of a base station, it may make repeated handoff requests in response to small changes in the environment. This problem, referred to as ping-ponging, is also greatly lessened by soft handoff. An exemplary process for performing soft handoff is described in detail in U.S. Pat. No. 5,101,501, entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
An improved soft handoff technique is disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM, which is assigned to the; assignee of the present invention and incorporated by reference herein. In the system of the ""261 patent, the soft handoff process is improved by measuring the strength of xe2x80x9cpilotxe2x80x9d signals transmitted by each base station at the mobile station. These pilot strength measurements are of assistance in the soft handoff process by facilitating identification of viable base station handoff candidates.
The base station candidates can be divided into four sets. The first set, referred to as the Active Set, comprises base stations, which are currently in communication with the mobile station. The second set, referred to as the Candidate Set, comprises base stations whose signals have been determined to be of sufficient strength to be of use to the mobile station but are not currently being used. Base stations are added to the candidate set when their measured pilot energy exceeds a predetermined threshold TADD. The third set, referred to as the Neighbor set, is the set of base stations which are in the vicinity of the mobile station (and which are not included in the Active Set or the Candidate Set). And the fourth set is the Remaining Set which consists of all other base stations.
In IS-95, a base station candidate is characterized by the phase offset of the pseudonoise (PN) sequence of its pilot channel. When the mobile station searches to determine the strength of the pilot signal from a candidate base station it performs a correlation operation wherein the filtered received signal is correlated to a set of PN offset hypotheses. The method and apparatus for performing the correlation operation is described in detail in U.S. Pat. No. 5,644,591, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEMxe2x80x9d, which is assigned to the assignee of the present invention and incorporated by reference herein.
The propagation delay between the base station and the mobile station is not known. This unknown delay produces an unknown shift in the PN codes. The searching process attempts to determine the unknown shift in the PN codes. To do this, the mobile station shifts in time the output of its searcher PN code generators. The range of the search shift is called the search window. The search window is centered about a PN shift hypothesis. A base station transmits to the mobile station a message indicating the PN offsets of base station pilots in its physical proximity. The mobile station will center its search window around the PN offset hypothesis.
The appropriate size of the search window depends on several factors including the priority of the pilot, the speed of the searching processors, and the anticipated delay spread of the multipath arrivals. The CDMA standards (IS-95) define three search window parameters. The searching of pilots in both the active and candidate sets is governed by Search Window xe2x80x9cAxe2x80x9d. Neighbor Set pilots are searched over window xe2x80x9cNxe2x80x9d and Remaining Set pilots over window xe2x80x9cRxe2x80x9d. The searcher window sizes are provided below in Table 1, where a chip is {fraction (1/1.2288)} MHz.
Window sizing is a trade-off between search speed and the probability of missing a strong path lying outside the search window.
The base station transmits to the mobile station a message which specifies the PN hypotheses that the mobile station should search relative to its own PN offset. For example, the originating base station may instruct the mobile station to search for a pilot 128 PN chips ahead of its own PN offset. The mobile station in response sets its searcher demodulator 128 chips ahead in the output chip cycle and searches for the pilot using a search window centered about the specified offset. Once the mobile is instructed to a search a PN hypothesis to determine the resources available for performing a handoff, it is critical that the PN offset of the destination base station pilot is very close in time to the directed offset. The speed of searching is of critical importance near base station boundaries because delays in completing the necessary searches can result in dropped calls.
In CDMA systems in the United States, this base station synchronization is achieved by providing each base station with a Global Positioning Satellite (GPS) receiver. However, there are cases where a base station may not be able to receive the GPS signal. For example, within subways and tunnels the GPS signal is attenuated to a degree that prohibits their use for timing synchronization of base stations or micro base stations. In addition, there are national agendas that discourage dependence upon the GPS signal for operation of critical services.
The present invention describes a method and system for providing timing synchronization in these circumstances where a fraction of the network is capable of receiving a centralized timing signal and achieving timing therefrom and a portion of the base stations are not capable of receiving the centralized timing signal. This situation is addressed in copending U.S. patent application Ser. No. 08/933,888, (the ""888 application), entitled xe2x80x9cMOBILE STATION ASSISTED TIMING SYNCHRONIZATION IN A CDMA COMMUNICATION SYSTEMxe2x80x9d, filed Sep. 19, 1997, now U.S. Pat. No. 5,872,774 which is assigned to the assignee of the present invention and incorporated by reference herein. In addition, the present invention describes a method and system for providing timing synchronization where no base stations rely on a centralized timing signal.
In the ""888 application, the slave base station attains synchronization with the reference base station through messages transmitted from and received by a mobile station in the soft handoff region between the reference base station and the slave base station. First, the round trip delay between the mobile station and the reference base station is measured by the reference base station. Next, the slave base station searches until it acquires the signal transmitted by the mobile station, referred to as the reverse link signal. In response to the acquisition of the reverse link signal, the slave base station adjusts its timing so that the mobile station can acquire its signal, referred to as a forward link signal. This step may be unnecessary if the timing error in the slave base station is not severe.
Once the mobile station acquires the signal from the slave base station, it measures and reports the difference between the amount of time it takes a signal to travel from the reference base station to it and the amount of time it takes a signal to travel from the slave base station to it. The last measurement necessary is a measurement by the slave base station of the time difference between the time it received the reverse link signal from the mobile station and the time it transmitted a signal to the mobile station.
A series of computations are performed upon the measured time values to determine the time difference between the slave base station and an adjustment of the slave base station timing is performed in accordance therewith. It should be noted that all of the measurements mentioned are performed during the normal operation of an IS-95 CDMA communication system.
The present invention is a novel and improved method and apparatus for synchronizing base stations in a wireless communication system. The present invention describes methods by which a wireless communication system keeps itself synchronized without an external reference. One method, disclosed in xe2x80x9cMOBILE STATION ASSISTED TIMING SYNCHRONIZATION IN A CDMA COMMUNICATION SYSTEMxe2x80x9d application Ser. No. 08/933,888, now U.S. Pat. No. 5,872,779 is to use the messaging of mobiles in handoff to determine the relative timing of pairs of base stations. Given the measured timing errors, the base stations"" timing is adjusted to maintain network synchronization.
When insufficient traffic is present in the network to maintain synchronization in this manner, other methods must be used. One approach involves making direct measurements of the timing between base stations. This is accomplished in one of two ways. The base may interrupt its transmissions on all sectors for a short interval during which it receives forward link signals from other base stations. Given knowledge of the other base station locations, time errors relative to all other base stations may be derived. Alternatively, a base station sends a short signal at high power in the mobile transmit band. The time-of-arrival of this signal is measured by the surrounding base stations and the time errors between pairs of base stations are computed.
In some cases, a base station may be isolated sufficiently from all other base stations in the network such that direct base-to-base measurement is not possible. In this case, a fixed dummy station is placed at a location in the handoff region between the isolated cell and another cell in the network. The fixed dummy station either performs measurements of base station pilots on command of the base and reports the timing information, or sends a burst transmission at a specified time to be measured by the base stations.